The Skybet loyalty thing doesn't seem to be good value to me. You have to do 4 or 5 £25 bets for a few weeks, then you might get a £50 FB. So for 4x£25 for 4 weeks, you'll probably make a loss of £20-40 (5-10% of £400) and then you might get a £50 FB.
I don't bother with their £5 club thing either, as I don't see much value in that. The £25 bet is going to take a loss of £2 probably, unless you go into arbing and then the £5 free bet is likely to make a loss of 50p-£1 if you put it on a double, or more if you put it on a single. So a profit of around £2-3 in total. Then add on a couple of mugs to make it look better and that's your profit gone.
All of that is taking into account avoiding excessively high odds or gubbing-likely arbs, of course, a bit more profit could be eked out if you took more risks on the bets you place.